The invention relates to a fuel nozzle such as a gas-air premixing burner for use in gas turbines, comprising an air swirler and annular burner tube surrounding a bluff centerbody. More particularly, the invention relates to a nozzle end configuration and to an adaptation for cooling the same.
Gas turbines for power generation are generally available with fuel nozzles configured for either “Dual Fuel” or “Gas Only”. “Gas Only” refers to operation burning, for example, natural gas and “Dual Fuel” refers to having the capability of operation burning either natural gas or liquid fuel. The “Dual Fuel” configuration is generally applied with oil used as a backup fuel, if natural gas is unavailable. The “Gas Only” configuration is offered in order to reduce costs as the nozzle parts and all associated equipment required for liquid fuel operation are not supplied. In general, fuel nozzles are designed to have “Dual Fuel” capability and the “Gas Only” version is a modification to the dual fuel design in which the liquid fuel parts, which include the oil, atomizing air and diluent water passages, are omitted from the nozzle and replaced with a component of similar size and shape, but without the internal features of the liquid fuel cartridge. This replacement component is known as a “Gas-Only Insert.” An example of a fuel nozzle configured for gas-only operation is illustrated in FIG. 1.
FIG. 1 is a cross-section through the burner assembly 10. The burner assembly is divided into four regions by function including an inlet flow conditioner 12, an air swirler assembly with natural gas fuel injection (referred to as a swozzle assembly) 14, an annular fuel/air mixing passage 16, and a central diffusion flame fuel nozzle assembly 18.
Air enters the burner from a high pressure plenum, which surrounds the assembly, except the discharge end which enters the combustor reaction zone. Most of the air for combustion enters the premixer via the inlet flow conditioner 12. The inlet flow conditioner includes an annular flow passage that is bounded by a solid cylindrical inner wall 20 at the inside diameter, a perforated cylindrical outer wall 22 at the outside diameter, and a perforated end cap 24 at the upstream end. In the center of the flow passage are one or more annular turning vanes 26. Premixer air enters the inlet flow conditioner 12 via the perforations in the end cap 24 and in the cylindrical outer wall 22.
After combustion air exits the inlet flow conditioner 12, it enters the swozzle assembly 14. The swozzle assembly includes a hub 28 and a shroud 30 connected by a series of air foil shaped turning vanes 32, which impart swirl to the combustion air passing through the premixer. Each turning vane 32 contains natural gas fuel supply passage(s) through the core of the air foil. These fuel passages distribute natural gas fuel to gas fuel injection holes 34 which penetrate the wall of the air foil. The fuel injection holes may be located on the pressure side, the suction side, or both sides of the turning vanes 32. Natural gas fuel enters the swozzle assembly 14 through inlet port(s) and annular passage(s) 36, which feed the turning vane passages. The natural gas fuel begins mixing with combustion air in the swozzle assembly, and fuel/air mixing is completed in the annular passage 16, which is formed by a centerbody extension 38 and a burner tube extension 40. After exiting the annular passage 16, the fuel/air mixture enters the combustor reaction zone where combustion takes place.
At the center of the burner assembly is a diffusion flame fuel nozzle assembly 18, which receives natural gas fuel through annular passage 42 and holes 44. In the center of this diffusion flame fuel nozzle is a cavity 46, which, as noted above, receives either the liquid fuel assembly to provide dual fuel capability or the gas-only insert. The gas-only insert 45 is shown in this example. In the dual fuel configuration, during gas fuel operation, the oil, atomizing air and water passages in this region are purged with cool air to block hot gas from entering the passages when not in use. When the nozzle is configured for gas only operation, cavity 46 must be substantially capped, as shown, at the distal end of the nozzle, to block hot combustion gas from entering the center region 46, which may result in mechanical damage due to the high temperature. A small amount of air passes through holes 47 in the end of the gas-only insert to cool and purge the tip of the gas-only insert.
Currently, the centerbody is cooled with air discharged directly into the recirculation zone 57 through orifices or passages 48 at the bluff face 63 of the centerbody. This air is sometimes referred to as curtain air. As schematically illustrated in FIG. 1, the curtain air stream 50 for cooling the centerbody conventionally feeds through a passage defined therefore in the swirler vanes 32, through annular passage 52 and, as mentioned above, exits through orifices or passages 48 at the end of the centerbody. However, this air does not have time to mix thoroughly before it reaches the flame.
Some fuel nozzle designs do not have a separate cooling air passage for the tip of the centerbody. These designs rely for cooling on air used to purge the diffusion fuel passages when fuel is not supplied to the diffusion fuel passages. In these designs, there is a risk of thermal distress during the transient transition between diffusion fuel flow and purge air flow.